1. Field of the Invention
The present invention relates to multiplexers. More particularly, the present invention relates to multiplexers that may be called upon in operation to shift between unrelated inputs.
2. Description of Related Art
Multiplexing means transmitting a large number of information units over a smaller number of channels or lines. A digital multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line. The selection of a particular input line is controlled by a set of selection lines. Normally, there are 2" input lines and n selection lines whose bit combinations determine which input is selected.
It is well known to those skilled in the art to combine several relatively low-bit-rate signals to form one relatively high-bit-rate signal to be transmitted over a high-frequency medium. Multiplexing is considered useful, generally, because it allows multiple signalling lines or channels that could not otherwise use the full capacity of a given data link to share that capacity, and therefore promote system efficiency.
In the multiplexing arts, it is possible that all channels will have identical bit rates. In such a case it is known to perform multiplexing on a bit-by-bit basis (known as bit or digital interleaving). It is also known in such a case to perform multiplexing on a word-by-word basis (known as byte or word interleaving). In such schemes, when the bit rate of incoming channels is not equal, the high-bit-rate channel may be allocated proportionately more slots. Further, it is evident that the minimum length of the multiplexer frame must be a multiple of the lowest common multiple of the incoming channel bit rates and, hence, such schemes are practical only when some fairly simple relationship exists among these rates.
Upon receipt at a destination terminal, the multiplexed digit stream must be divided and distributed to the appropriate output channel. To accomplish this, the destination or receiving terminal must be able to correctly identify each bit. This requires the receiving system to synchronize with the beginning of each frame, with each slot in each frame, and with each bit in each slot. This may be accomplished by adding "control bits" (e.g., framing bits and synchronization bits) to the data bits. As those skilled in the art are well aware, synchronization between all of the incoming channels and the multiplexer cannot be presumed. It is difficult to obtain synchronization even when all channels are nominally at the same rate. For example, it is known that small rises and drops in temperature can increase and decrease, respectively, pulse propagation velocity. If faster moving input pulses cannot be accommodated by the multiplexer, they must be temporarily stored. If, on the other hand, the rate of received pulses drops, the multiplexer may have vacant slots with no data. These vacant slots must be "stuffed" with dummy digits.
Obviously, if even in synchronously multiplexed systems data is rarely received at a synchronous rate, in pure asynchronous systems there are even more pronounced, although similar, problems to overcome to efficiently fill time slots. Heretofore, to multiplex asynchronous channels those skilled in the art have constructed so-called "statistical multiplexers", multiplexers which dynamically allocate time slots on demand. In such multiplexers, each input/output line has a buffer associated with it. For input, the multiplexer scans the input buffers, collecting data until a frame is filled, at which point it "sends" the frame. On output, the multiplexer receives a frame and distributes the slots of data to the appropriate output buffers.
All of the aforementioned types of multiplexers must be controlled. One of the common functions of multiplexers that must be so controlled is the function of switching between inputs, which may or may not be related. Switching between unrelated inputs, in particular, is a very important function to control well because it is frequently employed and because when it is performed poorly "glitches" occur at the multiplexer output.
Heretofore, there has not been developed a very simple, inexpensively constructable multiplexer control system that allows a multiplexer to switch between unrelated inputs without causing a glitch at the output. The lack of such a multiplexer control is a shortcoming and deficiency of the prior art.